The present invention relates to the field of an infrared zoom lens suitable for infrared rays over a wavelength range of 2.5 to 5.5 microns or 8 to 14 microns.
In the marketplace as of this writing there has been an arrival of products in the marketplace that can detect radiation between the ranges of 2.5 to 5.5 microns or 8 to 14 microns and through the use of an appropriate focal plane image detector array, sense and convert this radiation to digital imagery suitable for viewing on a display or monitor. This region of radiation is of interest to human observers since it primarily is the wavelength of heat radiation. As a result, any process or organism that generates heat such as humans self-generate the resultant imagery and that is clearly visible, even with zero lux of additional incident illumination on the subject. In the parlance of the marketplace this has been called “Night Vision” and/or “Infrared Cameras”.
As these technologies have matured over the past 10 years, the quality of the imagery has been improving and the price point of the devices in the marketplace has dropped approximately an order of magnitude. This has resulted in ever greater market penetration and a need for a variety of lenses to transfer the radiation from the subject into a clear crisp image onto the focal plane of the infrared cameras. Thus far the vast majority of the lenses are either two position lens systems (i.e. 50 or 200 mm focal length lenses) or fixed focal length lenses.
In the case of a mechanically compensated zoom system two moving subassemblies units are required to achieve the required optical performance which unfortunately calls for a relatively complex design. In contrast, in the case of an optically compensated system of the present invention, only the movement of one subassembly is required. The IR zoom lens device of Neil U.S. Pat. No. 4,632,498 has two independently moving parts to keep the lens device in focus. Furthermore, Neal requires at least one lens element having an aspheric surface to make his lens focus at all, which is more difficult to manufacture than a zoom system having no lens elements with aspheric surfaces as in the system of the present invention. Also he calls for the use of ZnSe without employing silicon, which is less desirable than the use of germanium and silicon.
Furthermore, Neil's design with his required aspheric surface, only allows for a good focus at four points (his table II, columns 5 and 6). In contrast, the present invention is a continuously in focus zoom lens device; that is, it's a truly optically compensated zoom lens.
Watanabe U.S. Pat. No. 6,091,551 employs the movement of positive lens groups, in contrast with the present invention employing single subassembly (carriage) movement of two lens groups together as a unit, yet providing continuous zoom motion with focus being maintained.